In this project, the Investigators plan to continue ongoing studies in the drug treatment of depressive illnesses, particularly nonendogenous depressions. Emphasis is placed on identifying patients who respond preferentially to either the tricyclic antidepressant, amitriptyline, or the monamine oxidase inhibitor, phenelzine. Using established clinical, biochemical and pharmacologic methods, we will carry out carefully controlled clinical trials in samples of outpatients with heterogenous depressive disorders. Patients having varying admixtures of endogenous and nonendogenous symptoms can be characterized by numerical placement along an unidimensional endogenous-nonendogenous continuum. Pharmacologic monitoring during the six weeks of double blind treatment includes the determination of plasma levels of tricyclic or MAO inhibitor drug, and platelet MAO inhibition. Phenelzine-treated patients are also assessed for acetylator phenotype and rate of acetylation of sulfapyridine is related to plasma phenelzine levels, MAO inhibition, and clinical drug effects. Single dose and steady state pharmacokinetic studies are conducted in patients treated with tricyclic and MAO inhibitor drugs and related to biologic variables. Multivariate statistical techniques, including discriminate function, factor and cluster analytic studies will be carried out utilizing both the self-rated Symptom Check List-90 (SCL-90) and Structured Depression Interview (SDI) data, and relating these to biologic, pharmacologic and clinical variables.